The core diagnostic criteria for autism spectrum disorder (ASD) are deficits in social interactions and communication. Another side to autism, though, is its impact on basic perception: people with ASD see, hear, and feel the world differently. One aspect of perception that poses difficulty for people with ASD is ignoring distractions - when performing visual tasks, increasing (but not all) evidence suggests that subjects with ASD experience more interference from irrelevant information than typical controls, becoming slower and less accurate in the face of distraction. A deficit in attentional selection might not only affect perception, but might further underlie the difficulties that individuals with ASD have in coping with everyday cognitive tasks in complex real-world environments. In the proposed research, we will pinpoint the specific stage(s) of attentional selection that are impaired in ASD using behavioral psychophysics. Further, we will test the involvement of the pulvinar and multiple-demand (MD) network, two brain regions known to support attentional selection, in ASD using functional magnetic resonance imaging (fMRI) and diffusion weighted imaging (DWI). The proposed experiments will address three specific questions: 1) Which attentional selection functions are impaired in ASD, and which are spared? 2) Which parts of the brain's attentional network show abnormal responses in ASD? 3) Is the anatomy of attentional areas, including their white matter connections, disrupted in ASD? The answers to these questions will advance our understanding of the cognitive and neural phenotype of ASD, and at the same time inform our understanding of visual attention and its brain basis in typical individuals. The broader objective of the proposed studies is to cultivate new expertise in studying subjects with autism, and in the design, analysis, and interpretation of structural imaging studies (including both morphometry and diffusion- weighted imaging).